The TransactionCoordinatorService should validate the incoming transaction number and session id against the latest transaction for the session. The logic should be as follows:

• If there exists a latest transaction for this session:
  • If txnNumber is the same as the latest on the session:
    • uassert that the coordinator has not yet received any votes or coordinateCommit
  • If txnNumber is higher than the latest on the session:
    • Call "tryAbort" on the latest coordinator for the session